Circuit for indicating when a battery is discharged



- D. C. TEDD May 27, 1969 CIRCUIT FOR INDICATING WHEN A BATTERY IS DISCHARGED Filed March 9, 1966 United States Patent 7 3,447,060 CIRCUIT FOR INDICATING WHEN A BATTERY IS DISCHARGED David Clifford Tedd, Great Barr, Birmingham, England, assignor to Eaton Yale & Towne Inc., Cleveland, Ohio, a corporation of Ohio Filed Mar. 9, 1966, Ser. No. 532,969 Claims priority, application Great Britain, Mar. 11, 1965,, 10,333/ 65 Int. Cl. H02j 7/04, 7/16 US. Cl. 320-48 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a circuit which, when connected to a battery, will give an indication when the battery is discharged.

A circuit according to the invention comprises in combination voltage-sensitive means operable when the voltage of the battery falls below a predetermined value for giving said indication, and current-sensitive means which inhibits the action of the voltage-sensitive means if the current drawn from the battery by a specified load or loads is above a value at which the voltage of the battery could fall below said predetermined value without the battery being discharged.

The accompanying drawing is a circuit diagram illustrating one example of the invention as applied to an industrial lift truck.

Referring to the drawing, there are provided a pair of terminals 11, 12 which in use are connected to the positive and negative terminals respectively of the battery 13. The terminals 11, 12 are bridged by a series circuit including a switch 8 which must be operated by the driver before the truck can be moved, a normally closed relay contact 9 and resistors 14, 10. A variable point on the resistor 14 is connected to the cathode of a Zener diode 15 having its anode connected to the terminal 12 through a resistor 16, and to the base of a transistor 17 through a resistor 18. The emitter of the transistor 17 is connected to the terminal 12, whilst its collector is connected to the base of a transistor 19, the emitter of which is connected to the terminal 12, and the base of which is connected to the terminal 12 through a resistor 20.

The terminals 11, 12 are further interconnected through a resistor 21 and a Zener diode 22 in series, the Zener diode 22 being bridged by a capacitor 23. A point intermediate the resistor 21 and Zener diode 22 is connected to the collector and base of the transistor 19 through the relay coil 24 and a resistor 25 respectively, the relay coil 24 being bridged by a diode 26 for conducting back and serving when energised to open the contact 9. The base of the transistor 19 is further connected to the collector of a transistor 27, the emitter of which is connected to the terminal 12 and the base of which is connected to a variable point on a resistor 29. The resistor 29 bridges a resistor 31 connected at one end to the terminal 12 and at its other end to the terminal 11 through 3,447,060 Patented May 27, 1969 a load 32 in series with the switch 8, the load 32 being powered by the battery 13.

The junction of the contact 9 and resistor 14 is connected to the terminal 12 through a microswitch 33 and a contactor coil 34 in series, the junction of the microswitch 33 and coil 34 being connected through a diode 35 to the junction of the Zener diode 15 and resistor 16. The microswitch 33 can be operated by the driver to energise the coil 34, which in turn operates contact 41 for operating the lift mechanism 40 of the truck.

In operation, when the battery is in charged condition the voltage across the Zener diode 15 is sufiicientrto cause it to conduct, and so current flows through the resistor 18 and the base and emitter of the transistor 17 to switch the transistor 17 on. Current flow through the resistor 25 is now diverted through the collector and emitter of the transistor 17, and so no base current flows in the transistor 19, which remains otf.

When the battery discharges, its voltage falls, and when its voltage falls below a value predetermined by the pos ition of the slider on the resistor 14, the Zener diode 15 ceases to conduct. The transistor 17 is now switched off, and current flow through the resistor 25 switches the transistor 19 on to energise the relay 24, which opens the contact 19 so that the warning lamp 7 is illuminated. Opening of the contact 9 also breaks the circuit to the Zener diode 15, which therefore cannot conduct again until the supply is removed. The resistor 21, Zener diode 22 and capacitor 23 stabilise the supply to the transistors, and prevent damage by transients.

The circuit thus far described is sensitive solely to battery voltage. For a given discharge rate, a battery may have a voltage per cell of 2.0 when the battery is charged, and 1.7 per cell when the battery is discharged, and so the slider movable over the resistor 14 can be set to operate so that the indication is given when the voltage per cell falls to 1.7. The indication will of course only be precisely accurate at the chosen discharge rate, because the battery voltage varies with the current drawn from it. However, for low values of current (for example below 200 amps), the battery voltage is substantially independent of current, and so the indication will be substantially accurate. The dilficulty arises where higher currents are drawn from the battery, for example when the truck is accelerated from rest. In this case, the battery voltage may well fall below 1.7 per cell, even though the battery is not discharged, but nevertheless the circuit so far described would give an indication that the battery is discharged. In order to overcome this problem, the transistor 27 and its associated components are included. The load 32 is constituted by the traction motor of the truck, and when the current through the motor is above a level predetermined by the position of the slider on the resistor 29, the transistor 27 can conduct, and acts in the same way as the transistor 17 to maintain the transistor 19 Oil". Thus, for a high current flow through the motor, the transistor 17 may be switched oil, but the transistor 27 will be switched on so the relay 24 is still not energised and the indication is not given. However, if the transistor 17 is switched off at any time when the current flowing through the motor, is below the predetermined value, the transistor 27 will also be oil, and so the relay 24 will be energized to give the indication. It will be apparent that the predetermined current chosen above which the indication will not be given must be a current above which the voltage of the battery starts to fall significantly.

The resistor 31 carries the full motor current, and obviously its resistance must be as low as possible. In the preferred example, the resistor 31 consists of a length of power cable, and the transistor 27 is chosen to operate at a very low voltage so that the power losses are kept to a minimum.

The arrangement disclosed prevents an indication from being given if the motor current is high, but in some trucks there may be other loads such as lift mechanism 40 which will take a current sufiiciently high to result in a false indication being given. In this case, further current sensing may be required, or alternatively the total battery current could be sensed instead of the current through a specified load. In most trucks, the traction motor is the only load which can result in any serious incorrect indication, although the current taken by the lifting mechanism 40 of the truck can also depress the battery voltage, but to a lesser extent than the traction motor. Separate sensing of the lifting mechanism 40 current may or may not be required, depending on the truck, but as will be explained the particular circuit shown caters for this problem in a different way.

The indication given by the lamp 7 is only incidental to the main indication in the preferred example, which is that the lifting mechanism 40 is put out of action. Thus, as soon as the coil 24 is energised, the contact 9 opens to break the circuit through the microswitch 33 to the contactor coil 34, so preventing the driver from using the lifting mechanism 40, whilst permitting him to drive the truck to a battery charging area. Current flowing through the lamp 7 is insufficient to energise the coil 34. The diode 35 performs three functions. Firstly, it maintains the transistor 17 conductive independently of the Zener diode 15 while the lifting mechanism 40 is being used, so that when the battery is discharged, the indication will only be given when the lifting mechanism 40 is in its lowered position. Secondly, it ensures that no false indication will be given as a result of the current drawn by the lifting mechanism 40. Thirdly, it ensures that the contact 9 only opens when the circuit through the coil 34 is dead, so that the contact 9 only has to break the small current flOWing through the resistors 14, 10.

The invention is primarily designed for use in industrial trucks, but can be used in any application where a straightforward measurement of battery voltage would be unsatisfactory because the voltage is from time to time affected by high currents drawn from the battery.

It will be appreciated that to de-energise the coil 24, one of the battery terminals must be disconnected. The coil 24 will also of course be de-energised when the truck is accelerated, but it will become energized again before the truck stops.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A circuit which, when connected to a battery, will give an indication when the battery is discharged, comprising in combination voltage-sensitive means in said circuit operable when the voltage of the battery falls below a predetermined value for giving said indication, and current-sensitive means in said circuit which inhibits the action of the voltage-sensitive means if the current drawn from the battery by a load is above a value at which the voltage of the battery could fall below said predetermined value without the battery being discharged and said current responsive means being operable continuously as long as such load current is being drawn from the battery.

2. A circuit as defined in claim 1, wherein the load includes an industrial truck traction motor.

3. A circuit as defined in claim 2, wherein said circuit includes a warning lamp indicator which is illuminated when said voltage sensitive means operates for indicating the battery is discharged.

4. A circuit as defined in claim 1, wherein said circuit includes means for preventing operation of the load upon operation of said voltage-sensitive means, thereby giving an indication when the battery is discharged.

5. A circuit as defined in claim 4, including means for preventing the indication from being given while the load is in operation.

6. A circuit as defined in claim 5, wherein the load includes an industrial truck traction motor and a lift mechanism, said circuit includes means for preventing operation of raising of the lift mechanism upon operation of said voltage-sensitive means, thereby giving an indication when the battery is discharged, said circuit includes means for preventing the indication from being given while the lifting mechanism is in operation and said circuit including a warning lamp indicator which is illuminated when said voltage means operates for indicating the battery is discharged.

U.S. Cl. X.R. 

